Method for producing double-reduced container stock having good resistance to corrosion and product produced thereby

ABSTRACT

A METHOD FOR PRODUCING DOUBLE-REDUCED CONTAINER STOCK INCLUDING PROVIDING STEEL CONTAINING NOT MORE THAN .03% CARBON, ABOUT 0.25% TO ABOUT 0.60% MANGANESE, ABOUT .014% TO ABOUT .025% SULFUR AND NOT MORE THAN ABOUT 0.20% COPPER WITH A RANGE OF ABOUT .06% TO ABOUT .20% COPPER PREFERRED. A WEIGHT RATIO OF NOT LESS THAN 12 MANGANESE TO ONE CARBON IS MAINTAINED IN THE STEEL. THE STEEL IS POURED INTO INGOT MOLDS, SLABBED, HOT ROLLED TO COIL FORM, COLD ROLLED TO AN INTERMEDIATE GAGE, ANNEALED AND DOUBLE-REDUCED COLD TO A LIGHT BASE WEIGHT. CONTAINER STOCK SO PREPARED HAS A CORROSION RESISTANCE OF NOT MORE THAN 60 MICROAMPERES PER SQUARE CENTIMETER WHEN TESTED BY A PHOSPHORIC ACID-BLUE DYE TEST (PABD), AND SUFFICIENT STRENGTH TO BE PROCESSED INTO CONTAINERS SUITABLE FOR CONTAINING HIGHLY CORROSIVE FOODS, CARBONATE BEVERAGES AND THE LIKE.

United States Patent METHOD FOR PRODUCING DOUBLE-REDUCED CONTAINER STOCK HAVING GOOD RESIST- ANCE T0 CORROSION AND PRODUCT PRO- DUCED THEREBY Carlton E. Roberts, Hilton N. Rahn, Jr., and Walter E. Reid, Jr., Bethlehem, Pa., assignors to Bethlehem Steel Corporation No Drawing. Continuation of abandoned application Ser. No. 689,000, Dec. 8, 1967. This application Dec. 21, 1970, Ser. No. 100,547

Int. Cl. (321d 7/14 US. Cl. 1482 6 Claims ABSTRACT OF THE DISCLOSURE A method for producing double-reduced container stock including providing steel containing not more than .03% carbon, about 0.25% to about 0.60% manganese, about .014% to about 025% sulfur and not more than about 0.20% copper with a range of about .06% to about .20% copper preferred. A weight ratio of not less than 12 manganese to one carbon is maintained in the steel. The steel is poured into ingot molds, slabbed, hot rolled to coil form, cold rolled to an intermediate gage, annealed and double-reduced cold to a light base weight. Container stock so prepared has a corrosion resistance of not more than 60 microamperes per square centimeter when tested by a phosphoric acid-blue dye test (PABD), and sufficient strength to be processed into containers suitable for containing highly corrosive foods, carbonate beverages and the like.

CROSS-REFERENCE TO RELATED APPLICATION This is a continuation of our application Ser. No. 689,- 000, filed Dec. 8, 1967, entitled Corrosion Resistant Container Steel Sheet, now abandoned.

BACKGROUND OF THE INVENTION This invention pertains to a method for producing container stock and more specifically to a method for producing double-reduced container stock characterized by having a combination of good resistance to corrosion and mechanical properties.

Modern trends in packaging corrosive foods and beverages in coated containers are directed to the use of stronger container stock of light base weights. Container stock commercially suitable for forming into containers must have a combination of good corrosion resistance, reasonably long shelf life, high strength, light base weight, good formability, flatness and good surface quality. The composition and the processing of the container stock from melting to forming the containers generally control the qualities of containers made therefrom.

The steels for the container stock are melted and teemed into standard ingot molds. Ingots, thus formed, are rolled into slabs which in turn are hot rolled to coil form to an intermediate gage, such as .085 inch. The coils are pickled and cold rolled to a gage such as .010 inch or to a gage which can be twice the gage of the container formed therefrom. The cold rolled coil is cleaned, annealed and is either temper rolled to obtain an extension of about 2% in the coil or is cold reduced by more than 30% in a double reduction step to produce black plate suitable for coating with tin or the like. It is known that in container stock having non-metallic inclusions, difiiculty is encountered with breakage when the coils are reduced in cross-sectional area by more than 30%. Consequently when the container stock is to be double reduced it is imperative that the steel be free of non-metallic in- 3,700,504 Patented Oct. 24, 1972 clusions such as sulfides, oxides and the like, on the surface and internally.

Heretofore container stock suitable for coated containers for corrosive foods and beverages were made from steels having a composition within the following chemical ranges:

Percent Carbon 0.03/0.l5 Manganese 0.20/0.60 Phosphorus Max./0.15 Sulphur Max/0.05 Silicon Max/0.02 Copper Max./0.20

The most corrosive foods, such as highly colored fruits and berries, and carbonated beverages, such as colas and grape soda, are packaged in containers made from type L steels having a composition with the following chemical ranges:

an adverse effect on the service or shelf-life of coated containers. Copper decreases or increases the corrosion resistance of the containers depending upon the product packaged therein. However, the detrimental effects of copper overshadow any beneficial effects derived therefrom. For this reason the most corrosive foods and beverages are packaged in containers made from steels containing as little copper as possible, e.g. type L steels of the prior art. The production of such steels require good quality control of the raw materials used in the manufacture thereof since copper is not removed during the steel refining operations. Although the specifications of the L- type steels include a maximum carbon content of .12%, from a practical viewpoint the steels are not melted to a carbon content below about .03% to .04% because of the strength requirements of the finished containers and the difiiculty in handling thin sheets of steel which are soft because of low carbon contents.

One prior art method to avoid this restriction of copper content is shown in US. Pat. No. 3,367,751 issued Feb. 6, 1968 to James W. Halley et al. which is directed to the addition of larger than usual amounts of sulphur so as to form sulfide inclusions in the steel to improve the corrosion resistance thereof. However, it has been found that sulphur in excess of .03% adversely alfects tin plate surface quality. The appearance of a large number of non-metallic sulfide inclusions on the surface of the steel results in excessive number of rejections because of poor surface quality. Then, too, the non-metallic inclusions are foci for failure during forming of the ends of the containers, also resulting in a larger than usual number of rejections.

As noted previously, container stock is generally slabbed, hot rolled, cold rolled and annealed to prepare the stock prior to applying the protective coating thereon. The cold reduction of the stock determines the uniformity of the gage and flatness of the final coated stock. The lighter gage container stock is double reduced, that is, passed through a high speed tandem mill, wherein the stock is reduced by about 30% to about 50%. Unfortunately, such cold reduction of container stock has an adverse effect on the corrosion resistance of the containers made therefrom, resulting in short shelf-life of the coated containers. As a result, the steel compositions mentioned have a limited shelf-life for the most corrosive foods and beverages.

In an effort to obtain stiffness in container stock, prior art practices, as exemplified by US. Pat. No. 2,393,528 issued Jan. 22, 1946, to Norman P. Goss, are directed to the additions of alloying elements such as silicon, aluminum, copper and manganese to a stock containing .10% to .12% carbon. The container stock is subjected to a heavy reduction of about 15% for maximum stiffness in the cold rolled coil. The lighter gage container stock thus prepared has sufiicient strength to be used as containers. While Goss improves the stiffness of his container stock he makes no mention of improved corrosion resistance in combination with the improved stiffness.

SUMMARY OF THE INVENTION The principal object of this invention is to provide a method for producing double-reduced corrosion resistant container stock which can be provided with a protective coating and which is made by standard melting and processing techniques.

It is another object of this invention to provide a method for producing container stock characterized by having good strength and good resistance to corrosion and having a low carbon content, a low sulfur content, a high copper content and a manganese content predicated on a weight relationship with the carbon content.

It is another object of this invention to provide a double-reduced container stock having low carbon content and a manganese content predicated on a weight relationship with the carbon content.

It is another object of this invention to provide a double-reduced container stock which is suitable for use as a container for highly corrosive foods and carbonated beverages.

Other objects of this invention will become apparent as the description of the invention proceeds.

Broadly, the method of the invention includes providing a melt of steel, which is to be used for container stock, having low carbon content, a manganese content predicated on the carbon content, which container stock is double reduced to light base weight having the combination of good strength and corrosion resistance.

DESCRIPTION OF THE PREFERRED EMBODIMENT The container stock of the invention is prepared from steel melted to the following chemical ranges:

Carbon (percent) Max./0.03 Manganese (percent) 0.25/0.60 Sulphur (percent) 0.014/0.025 Copper (percent) Max./O.20 Mn (percent)/C (percent) 12/1 the remainder substantially iron and incidental impurities. Of course, it will be understood that residual elements such as silicon, aluminum, arsenic, phosphorus, nitrogen, chromium, nickel and molybdenum Will be kept as low as possible, consistent with the requirements of this type of steel. It has been found that the manganese content is predicated on the carbon content of the steel on a weight ratio of not less than 12 parts of manganese to 1 part of carbon.

The steel can be melted by any of the standard melting methods in a basic open hearth furnace, a basic oxygen furnace and the like provided that compositions are adjusted in the liquid state. The steel is cast into standard ingot molds and processed in the usual manner, that is, slabbed, hot rolled to coil form on a continuous hot strip mill, cold reduced on a multi-stand high speed tandem mill to black plate gage and annealed. The annealed black plate is double reduced by about 30% to about 60%, producing a container stock having the necessary mechanical strength, corrosion resistance and which can be coated with tin and the like, to produce containers for highly corrosive foods and carbonated beverages. Corrosion resistance and formability of the container stock are not adversely affected by double-reduction. It must be understood that the terms double reduced and duo-reduced are synonymous and that a double reduction step produces light base weight stock having a thickness of between about .0055 of an inch to .0068 of an inch.

Container stock which is to be used to manufacture containers for highly corrosive foods and carbonated beverages is tested by a standard PABD test. The PABD test as developed by container manufacturers is herein incorporated by reference to Halley et al., supra, column 4, lines 34-53. While the reference states that a corrosion rate of 20 microamperes per square centimeter is indicative of excellent corrosion resistance, a corrosion rate of not more than 60 microamperes per square centimeter has been set as a standard by container manufacturers.

Type L steels presently made into containers for highly corrosive foods and beverages are not entirely satisfactory when double reduced to light base weight stock now desired. The amount of cold working produces container stock with a yield strength in the range of about 75,000 p.s.i. to about 90,000 p.s.i. However, the double reduction of the container stock adversely affects the corrosion resistance of the steels. Several L-type steels used for container stock were melted, slabbed, hot rolled to a thickness of .080 of an inch to .100 of an inch, cold reduced to a thickness of .010 of an inch and annealed. The cross-sectional area of the container stocks was double reduced about 40% in a multi-stand high speed tandem mill to a thickness of about .006 of an inch. The doublereduced container stock was tested according to the standard PABD test mentioned above. The following Table I shows the chemical composition and the results of PABD tests:

TABLE I.CHEMICAL COMPOSITION AND PABD TESTS ON DOUBLE-REDUCED L TYPE STEELS Chemical composition (percent) PABD tests, Test No. 0 Mn P S Cu Mn/C microamps/crn.

It will be noted that the container stock which is double reduced to decrease the cross-sectional area by about and having a manganese to carbon ratio of less than 12 did not meet manufacturers standard of not more than microamperes per square centimeter. Therefore, the above container stocks are not acceptable to container manufacturers for producing containers for highly corrosive foods and carbonated beverages.

Unexpectedly we have found that double-reduced container stock which has good corrosion resistance and the desired strength, which is a yield strength within the ranges of about 80,000 p.s.i. to about 90,000 p.s.i., can be made by melting steel having not more than .03% carbon, a manganese content predicated upon the carbon content whereby the manganese to carbon ratio is not less than about 12 to 1, a relatively low sulfur content of not more than about .025 and a relatively high copper content up to about .20%. The container stock made from the above steels can be double reduced to light base weights, for example about .0055 of an inch to about .0068 of an inch, without adversely affecting the corrosion resistance of the container stock and still maintain strength sufficient to be processed into containers for highly corrosive foods and carbonated beverages.

Table II below lists several steels of this invention which can be used for light base weight container stock. The manganese/carbon ratio and the results of testing the container stock by the standard PABD test are shown. The container stock was processed by standard methods including double reducing the container stock by about 40% in cross-sectional area to light base weights:

TABLE II.CHEMICAL COMPOSITIONS AND PABD TESTS ON DOUBLE-REDUCED STEELS OF THE INVENTION Chemical composition (percent) PAB D tests,

Test No. 0 Mn P S Cu Mn/C microamps/cm.

The balance in each steel is essentially iron and incidental impurities.

Where a range of PABD test values is shown, it indicates the extremes of two or more tests. The tests indicate that the container stocks of Table II which are double reduced to reduce the cross-sectional area by about 40%, can be used to produce containers for highly corrosive foods and carbonated beverages. The container stocks all show a value of not more than 60 microamperes per square centimeter, which criterion is required by the container manufacturers.

It will be noted that the container stocks of the invention tested above are made with a relatively wide range of copper while sulphur is maintained at the low levels compatible with optimum surface quality. Carbon is maintained at low levels and manganese is maintained at levels to maintain the manganese to carbon ratio of not less than 12. The container stock is produced by a doublereduction step which results in a cold reduction in the cross-sectional of about 40% without detrimental effects on the corrosion resistance of the containers made therefrom.

In a specific example of the invention, steel listed as Test No. 8 in Table II was melted in a basic open hearth furnace. The steel had the following chemical composition:

Percent C .006 Mn .33 P .008 S .024 Cu .034 Mn/C 55 The steel was teemed in 23" x 41" ingot molds. The ingots were rolled into slabs which were 7 inches in thickness. The slabs were hot rolled on a continuous hot strip mill into coil form having a thickness of about .080 of an inch to about 0.10 of an inch. The coils were pickled and then cold rolled on a multi-stand high speed tandem mill to a thickness of about .010 of an inch. The coils were batch annealed within a temperature range of 1200' F. to 1250 F. in an inert atmosphere. The coils were double reduced cold on a multi-stand high speed tandem mill to about .006 of an inch in thickness, which is a reduction of 40% in the cross-sectional area. Specimens of the container stock were tested by the aforementioned PABD test. The container stock had a PABD of 20-35 microamperes per square centimeter.

In another specific example of the invention, steel listed as Number 7 in Table II for container stock was melted to the following chemical composition:

The steel Was continuously cast into slabs having a thickness of about 8 inches. The slabs were processed hot to a thickness of about .100 of an inch. The coil was pickled and cold rolled to a coil having a thickness of about .010 of an inch. The coil was batch annealed within a temperature range of 1200 F. to 1250 F. in an inert atmosphere. The annealed coil was double reduced cold on a multi-stand high speed tandem mill to a thickness of about .006 of an inch, which is a reduction of 40% in the cross-sectional area. Specimens of the container stock were tested by the aforementioned PABD test and showed the container stock to have a PABD of 35-60 microamperes per square centimeters.

All percentages referred to herein are on a weight basis unless otherwise noted in the specification and claims.

We claim:

1. In a method for producing light base weight container stock having improved resistance to corrosion by highly corrosive foodstuffs. and beverages, wherein steel is melted, tapped, teemed into ingot molds, stripped, slabbed, hot rolled to coil form, cold rolled to an intermediate gage, annealed and double-reduced to said light base weight and tinned, the improvement comprising melting a steel consisting of not more than about .03% carbon, from about 25% to about .'60% manganese, about .014% to about .025% sulfur and up to about .20% copper, the remainder substantially iron and incidental amounts of elements usually associated with steels of this type, the manganese content being in the weight ratio of not less than 12 to 1 with the carbon content.

2. The method of claim 1 in which the melted steel contains about .014% to about .025% sulfur and about .06% to about .20% copper.

3. The method of claim 1 in which the steel is characterized in the double-reduced condition as having a corrosion resistance of not more than 60 microamperes per square centimeter when tested by the PABD test.

4. A double-reduced and tinned low-carbon steel container stock suitable for use as a container for highly corrosive foods and beverages and consisting of not more than about 0.03% carbon, about 0.25% to about 0. 6% manganese, about 0.014% to about 0.025% sulfur, and up to about 0.20% copper, the remainder substantially iron and incidental amounts of elements usually associated with steels of this type, in which the manganese content and carbon content are regulated to obtain a weight ratio of not less than 12:] respectively.

5. The low-carbon container stock as claimed in claim 4 in which the container stock is characterized by having a corrosion resistance of not more than 60 microamperes per square centimeter when tested by the PABD test.

6. A double-reduced and tinned low-carbon steel container stock suitable for use as a container for highly corrosive foods and beverages and consisting of not more than about 0.03% carbon, about 0.25% to about 0.60% manganese, about .014% to about .025% sulfur, the copper content is between about .06% and .20%, and the manganese content and carbon content are regulated to obtain a weight ratio of not lesst than 12:1 respectively.

References Cited UNITED STATES PATENTS 3,264,144 '8/1966 Frazier 14812 3,282,685 11/1966 Mayer 75--125 3,368,886 2/19'68 Muta 148-12 X OTHER REFERENCES The Making, Shaping, and Treating of Steel, United States Steel, Eighth Edition, 1964, pp. 950 and 951.

L. DEWAYN'E RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R. 

